Well, if you cut 'n' pasted your code into your post, then I'd say you got a typos.
first: you don't need parentheses ("()") with calls of echo,
echo "hello";
works fine also.
Second: choose a somewhat more defining subject for your post next time - one should be able to understand the main problem from reading the subject.
Now, here's the code as I think it should do:
require("html_functies.php3");
$sql = "Insert into koper(naam, prijs, email, tel, verkoperid) values('$naam', '$prijs', '$email', '$tel', '$verkoperid')";
if (!($link = mysql_pconnect($hostname, $user, $pass)))
{
echo("Fout met verbinding maken met database");
exit();
}
if(!mysql_select_db($database, $link))
{
echo("Fout met Database selecteren"); // <- this missing second quote
// might have caused the error
exit();
}
$voer_uit = mysql_query($sql, $link);
if(!$voer_uit)
{
echo("Fout met sql uitvoeren");
exit();
}